(1) Field of the Invention
This invention relates in general to solid state imaging devices, and in particular to such devices which employ transparent electrodes.
(2) Description Relative to the Prior Art
Solid state imaging has become a reality with the advent of silicon charge handling devices such as charge-coupled and charge-injection devices.
In a charge handling device, typically, depletion regions, or potential wells, are formed about 1 to 10 microns below surface electrodes thereof; and imaging light, entering the bulk silicon of the device, generates minority carriers in proximity to such electrodes. This precludes imaging onto the non-electrode-carrying, or back-, side of the device without first thinning the bulk silicon forming such device, such bulk being generally 200 to 400 microns in thickness before thinning.
Rather than backside-thin a silicon charge handling wafer, the trend clearly is toward the use of transparent electrodes as vehicles for allowing imaging light to generate minority carriers in charge-depleted wells. To date, several forms of transparent electrodes have been considered for silicon imaging devices: highly doped polycrystalline silicon electrodes; extremely thin gold or silver electrodes; wide bandgaps semiconductor electrodes such as electrodes of In.sub.2 O.sub.3 or SnO.sub.2. All such electrodes leave something to be desired for a variety of reasons: polycrystalline silicon electrodes are virtually opaque to blue light; gold and silver electrodes must be extremely thin, and of uniform thickness, a matter which leaves them fragile, hard to make and, even if perfectly made, they exhibit non-uniform absorption in the visible part of the spectrum; wide bandgap semiconductor electrodes such as doped In.sub.2 O.sub.3 and SnO.sub.2, although having good optical transmission properties, are unstable at high temperatures.
Typical of prior art techniques for providing a silicon imaging device having transparent electrodes is that described in U.S. Pat. No. 3,941,630, issued Mar. 2, 1976. U.S. Pat. No. 3,941,630, aside from describing the use of doped SnO.sub.2 or In.sub.2 O.sub.3 electrodes, describes some of the very same considerations which have given rise to the present invention.
Rather than any of the prior art transparent electrodes, the invention, as will appear below, concerns the application of silicon carbide (SiC) for electrode purposes. SiC has characteristics which are complementary to the fabrication of silicon imaging devices: SiC is intrinsically compatible with the silicon which forms the device; SiC is inert; and SiC can serve as a diffusion barrier. Several examples of prior art teachings have addressed various uses of SiC: U.S. Pat. No. 3,400,309, issued Oct. 3, 1968, discusses the epitaxial growth of monocrystalline SiC "layers", as well as the "insulating" qualities of SiC layers. U.S. Pat. No. 3,577,285, issued May 4, 1971, discusses the doping of SiC to alter its conductivity. U.S. Pat. No. 3,504,181, issued Mar. 31, 1970, although indicating the light transmissiveness of SiC, indicates the difficulty of trying to etch SiC. Indeed, Japanese Journal of Applied Physics, Volume 14, 1975, No. 11, pages 1833 and 1834, indicates that grown (monocrystalline) SiC films are (1) transparent (although appearing brownish yellow), and (2) even without doping, are of low resistivity (contrasted with the teaching of U.S. Pat. No. 3,400,309).
Having the essential qualities for imaging device electrodes, it may appear puzzling that SiC has never been employed for transparent electrode purposes. However, if one considers the following table taken from page 284 of Heterojunctions and Metal-Semiconductor Junctions, by A. G. Milnes and D. L. Feucht, Academic Press, 1972, Library of Congress Catalog Card No. 79-127693, one might surmise why SiC electrodes have, apparently, been ruled out for imaging device electrodes . . . "SiC electrode patterns cannot be etched conveniently". (Indeed, the etching technique of U.S. Pat. No. 3,504,181 involves "molten" Na.sub.2 OH plus NaOH which, obviously, would be unattractive in the manufacture of silicon imaging devices, since it would attack both photoresist and SiO.sub.2 and would also lead to sodium contamination of the device, an undesirable condition.
__________________________________________________________________________ ETCHES AND THEIR EFFECTS ON SEMICONDUCTORS.sup.a Semi- White- Methanol Chromic Other Conductor HCl H.sub.2 SO.sub.4 HNO.sub.2 etch.sup.b bromine.sup.c NaOCl.sup.d NaOH.sup.e H.sub.2 O.sub.2 acid etches __________________________________________________________________________ Ge N N S V N S M S N Si N N N V N N M.sup.f N N SiC N N N N N N N N N GaAs S, N S S, N V S, M S S N N GaN N N N N N N S.sup.f N N GaP N N N S S S.sup.g N N N GaSb N.sup.g N V V M S N N S M.sup.h InAs M.sup.i N M M M N N N S M.sup.j InP V, M N N N M N S N N InSb N.sup.k, S.sup.g N V V M N N N N M.sup.l CdS M N V S S N N N S CdSe S.sup. i, m N V V M N N N S.sup.m M.sup.h ZnS S N N N S N N N S.sup.g ZnSe S.sup.g N M.sup.m M V S.sup.k, M.sup.g M.sup.g V S.sup.g ZnTe S.sup.i N M.sup.m V M N V N S __________________________________________________________________________ .sup.a All solutions are at room temperature and concentrated unless otherwise noted. V = etches material vigorously; M = etches material moderately; S = etches material slightly; N = does not perceivably etch material .sup.b 1 HF:4 HNO.sub.3. .sup.c Approximately 5% Bromine.? .sup.d 30% NaOCl. .sup.e Approximately 20% NaOH and warm 40.degree.-50.degree. C. unless noted otherwise. .sup.f 50% NaOH and hot 90.degree.-100.degree. C. .sup.g Hot 90.degree.-100.degree. C. .sup.h Etch: 1 HF:2 HNO.sub.3 :1 CH.sub.3 COOH. .sup.i 20.degree. to 100.degree. C. .sup.j Etch: 1 HF:5HNO.sub.3 :1 CH.sub.3 COOH. .sup.k 20.degree. C. .sup.l Etch: 2 HF:1 HNO.sub.3 :1 CH.sub.3 COOH. .sup.m Tends to form surface layer which inhibits etching. .sup.n Etch: (12 K.sub.2 Cr.sub.2 O.sub.7 sat. sol.: 4 H.sub.2 SO.sub.4) (3 HCl) at room temp. Rinse: 1 Na.sub.2 S.sub.2 O.sub.4 : 1 NaOH: 3 H.sub.2 O at 85.degree. C.